1. Field of the Invention
The present invention relates to a method of preparing light-sensitive silver halide grains and, particularly, to the method of growing crystal grains of light-sensitive silver halides.
2. Description of the Prior Art
Light-sensitive silver halide grains are usually prepared by mixing two or more aqueous solutions of inorganic salts (a silver salt such as silver nitrate and at least one halide such as KBr, NaCl, KI, NH.sub.4 Br and the like) in the presence of a protective colloid such as gelatin. Hitherto, when silver halide grains having uniform size and form are desired, the double jet method is carried out, in which two or more aqueous solutions of inorganic salts are simultaneously added to a reaction syste at a nearly constant rate. However, this method is inefficient because it requires a very long time for crystal grain growth. Therefore, the following improvements have been made in the method.
(1) The increase in grain size can be attained by the addition of a large amount of silver halide solvent such as ammonia, thiocyanates, thioethers or the like.
(2) The increase in grain size can be attained by increasing the addition amounts of each of the aqueous solutions of inorganic salts to such a rate that the addition amount varies proportionately to the rate of increase in the total surface area of silver halide grains produced or less than the rate of increase of the total surface area of silver halide. In accordance with the latter practice, the flow rate of the aqueous solution is allowed to increase according to the expression of at.sup.2 +bt+c, wherein t is time required for precipitate formation, and a, b and c are constants determined depending upon conditions for precipitate formation such as temperature and others. These improvements are described in detail in Japanese Patent Publication Nos. 36890/73 and 16364/77 (corresponding to U.S. Pat. Nos. 3,650,757 and 3,672,900, respectively), and the collection of preliminary reports published in autumn meeting of Japan Photographic Society in 1977, page 1.
The former method (1) suffers from the disadvantages that the formation of crystal grains is accompanied by side reactions of the silver halide solvent used, and it is difficult to change the pH in the vessel over a wide range because, for instance, when ammonia is employed as the silver halide solvent and it is desired to lower the pH by the addition of an acid, the action of the ammonia as the silver halide solvent is reduced.
The latter method (2) also has the defects that (a) the addition amounts must be sharply increased in the final stages of grain formation with the result that there is an abrupt increase in the quantity of liquid in a reaction vessel and it is difficult to achieve uniform stirring, and (b) the density of crystal grains (the number of crystal grains present in a volume of one liter) in the final stages of grain formation decreases and consequentially the critical rate of crystal grain growth decreases, the renucleation occurs easily and the size distribution is apt to widen, as described hereinafter, and (C) an elaborate apparatus capable of sharply varying the flow rate with great precision is required but is not easy to produce. In actual apparatuses, the flow rate can be varied within a limit of several ten-fold at the most. In particular, when the quantity of flow becomes very small, it is difficult to achieve the homogeneous mixing of solutions added.